Lasers can be attractive sources for imaging systems for a variety of reasons. For example, they can be very bright sources. In addition, the highly temporally coherent (i.e., monochromatic) light that is typically emitted by lasers obviates the need to correct for chromatic aberrations. This is particularly advantageous at shorter wavelengths, such as those within the ultraviolet spectrum, where fewer choices of optical materials are available to correct for chromatic dispersions. However, laser sources also tend to be highly spatially coherent. Such highly coherent sources produce spatially random interference patterns or "speckle" as they are called, particularly accompanying reflections from or transmission through rough surfaces.
Speckle is a type of noise that obscures optical information presented in such forms as interference patterns or object images. Reported approaches to speckle reduction can be organized into five categories:
1. Average over many speckle patterns generated by a quasi-extended source, i.e., effectively reduce spatial coherence, PA1 2. Average over many speckle patterns generated by a varying wavelength source, i.e., effectively reduce temporal coherence, PA1 3. Average over many speckle patterns generated by spatially sampling different portions of an optical field, PA1 4. Average over many speckles within a detector area or time-varying characteristics of the individual speckles, and PA1 5. Digital image processing.
Our invention relates most closely to the first listed approach but can be used in combination with features of one or more of the other approaches to further enhance speckle reduction. For example, our system is effective with stationary components and an invariable nearly monochromatic source, but could possibly function even better with moving components or a varying wavelength source. Cost and added complexity are also considerations of such combinations.
The nearly monochromatic source contemplated for this invention is a pulsed source, such as an excimer or a solid state laser, that emits a series of pulses that are much shorter than an integration interval (response time) of a detector. In addition, the coherence length of the pulses from the nearly monochromatic source is preferably much shorter than a maximum pulse length corresponding to the detector's integration interval.
Within an optical processing system of this sort, we propose to accomplish speckle reduction by a combination of temporal division and spatial aberration. Each of the pulses is divided into a series of overlapping pulselets having coaxial wavefronts and the same length "I.sub.P " as the individual pulses but staggered in time with respect to each other, preferably through increments greater than a coherence length ".lambda..sub.C ". The total length through which the pulselets are staggered increases a composite length "L.sub.T " of the overlapping pulselets to at most fill the integration interval of the detector. In addition, each of the pulselets is spatially aberrated with respect to other of the pulselets that are spaced beyond the coherence length ".lambda..sub.C ". Accordingly, each pulselet temporarily produces a unique speckle pattern that combines with the different speckle patterns of all the other pulselets spaced beyond the coherence length ".lambda..sub.C " to reduce by averaging the speckle effects produced by the nearly monochromatic light.
The improvement can be measured in terms of speckle contrast "C", which can be expressed mathematically as follows: ##EQU1##
where ".sigma..sub.I " is a standard deviation of intensity across a speckle pattern and "I.sub.M " is the mean intensity of the speckle pattern. The speckle pattern produced by a fully coherent source has unit contrast (C=1).
The speckle contrast "C" can also be related to a number "N" of independent speckle patterns of intensity "I" (i.e., random speckle patterns that combine incoherently) in accordance with the following relationship: ##EQU2##
If all of the speckle patterns are produced by beams having the same energy, the equation for contrast "C" reduces to: ##EQU3##
Increasing the number "N" of independent speckle patterns reduces speckle contrast "C". However, due to the exponential nature of the relationship, reductions in speckle contrast "C" much below 1% occur quite slowly with increases in the value of "N". For example, only 25 independent speckle patterns are required to reduce speckle contrast to 20%, 10,000 independent speckle patterns are required to reduce speckle contrast to 1%, and 1,000,000 independent speckle patterns are required to reduce speckle contrast to 0.1%.
In practice, the number "N" of independent speckle patterns is generally limited by the composite length "L.sub.T " of the overlapping pulselets required to fill the integration interval of the detector and by the coherence length ".lambda..sub.C " of the monochromatic light as follows: ##EQU4##
Shorter composite lengths "L.sub.T " reduce the number "N" of possible independent speckle patterns. Composite lengths "L.sub.T " that extend beyond the integration interval of the detector waste illuminating power and do not contribute any more independent speckle patterns for averaging within the integration interval.
Temporal divisions of the pulses that are variously spatially aberrated with respect to each other within the coherence length ".lambda..sub.C " can contribute to differences between the "N" number of independent speckle patterns. An intensity pattern generated by pulselets overlapping within the coherence length ".lambda..sub.C " is found by summing their respective fields vectorially having regard to phase differences, prior to squaring these sums. While the resulting speckle pattern can be quite random, speckle contrasts "C" up to unity are still possible. However, an intensity pattern generated by pulselets overlapping beyond coherence length ".lambda..sub.C " is found by squaring their individual fields and then summing these squares, which result in an overall averaging that yields a reduced speckle contrast.